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Perception of Colour

Colours: A beautiful aspect of life Colours: A beautiful aspect of life Colour has a profound effect on the feelings of a human being. In clothing, interiors, landscape and even natural light, a colour can change a confusion to clarity, a fear to confidence. An object's response to a colour may be influenced by a number of factors such as the body’s need for a specific colour, etc. Lets discuss the factors that influence the colour of an object.

Learning objectives

After completing the topic, the student will be able to:

  • Expand the concept of absorption and dispersion of light based on daily life scenarios.
  • Differentiate between transparent and opaque objects based on the properties of light; also explore them based on the relevant daily life experiments.
  • Explore about selective reflection and transmission of light by the objects using everyday aspects of science.
  • Examine and explore the components of white light and illustrate the primary and secondary colors of light by incorporating them in getting other beautiful colors.
Absorption of Light Absorption curve showing I/Io versus wavelength The curve peaks for green color, which means that the material reflects green color.
Absorption of light

When light enters a transparent material some of its energy is dissipated as heat energy, and it thus loses some of its intensity.  When this absorption of energy occurs selectively for different wavelengths of light, the light that gets transmitted through the material will show only those wavelengths of light that are not absorbed.  The transmitted wavelengths will then be seen as color, called the absorption color of the material.

For example, if we measure the intensity of light I0, for each wavelength before it is transmitted through a material, and measure the intensity I, for each wavelength after it has passed through the material and plot I/I0 versus wavelength we obtain the absorption curve for that material as shown here. The absorption curve (continuous line) for the material in this example shows that the light exiting the material will have a yellow–green color, called the absorption color.

An opaque substance would have an absorption curve such as that labeled "Dark", i.e., no wavelengths would be transmitted. Sunlight, on passing through the atmosphere has absorption curve as shown. Thus we see it as white light, since all wavelengths are present.

VIBGYOR Dispersion of light through a prism In a prism, material dispersion (a wavelength-dependent refractive index) causes different colors to refract at different angles, splitting white light into a rainbow. The splitting of a ray into its component colours is known as dispersion of light and the band of colours is known as a spectrum.
Formation of a rainbow
Formation of a rainbow - dispersion A rainbow is formed when white light is dispersed through raindrops.
Dispersion of light

Sky is blue and leaves are green. To the physicist, the colors of objects are not in the substances of the objects themselves or even in the light they emit or reflect. Color is a physiological experience and is in the eye of the beholder. So when we say that light from the sky is blue, in a stricter sense we mean that it appears blue. Many organisms, including people with defective color vision, will not see the sky as blue at all.

When a beam of sunlight or white light falls on a prism, we will observe that it gets separated as different band of colours. These colours are V(violet), I(indigo), B(blue), G(green), Y(yellow), O(orange), R(red) ––– VIBGYOR. This shows that the beam of white light is actually a composite of seven different colours. Each colour has a particular wavelength and frequency.

This phenomenon of dispersion was first discovered by Newton. He also passed each individual coloured light through another prism, and concluded that each colour has no other component. Thus a ray of yellow light is composed of yellow colour only. Similarly a ray of blue or red light will have only blue or red colours. A white light is a mixture of all seven colours.

When light rays fall on a material, they can undergo any of the phenomenon like transmission (refraction), absorption or reflection. If the light falling on the material passes through it completely, we say that the material is transparent. If all the light falling on an object is absorbed, then it will appear black to our eyes. If all the light is reflected (no absorption), the object will appear in the same colour as the colour of the light beam. For example if white light is falling on the object then the object will appear white.

Exercise:Try it yourself

Try and do a simple experiment. Take a square of black paper and another square of white paper. Keep them side by side near a wall. Make the room dark. Now flash a torch on the two pieces so that the light scattered will hit the wall. What will you observe? You will see that the white square is reflecting a lot of light whereas the black square is not. This simple experiment demonstrates the fact that black colour absorbs all the light falling on it and that white colour reflects all the light falling on it.

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